Computer components typically run off of direct current, or "DC". Not only this, but different components within the PC actually require different DC voltages. Our cities' power grids provide alternating current, or AC. So, for a computer to receive the electricity it needs to function, we not only need to convert AC to DC power, but we also need to convert the AC into several different DC voltages.

To do this, computers use a "power supply", often called PSU for short (the "U" stands for "Unit".) Computer power supply units are of the "switch mode" variety and are capable of providing several different DC voltages at very high current while still remaining relatively small in size and efficient.